During a combustion, unlike sulfur oxides, acids and heavy metals, the emissions of which are intrinsically linked to the content of sulfur, of Cl (chlorine), Br (bromine), F (fluorine), I (iodine), and of heavy metals of the fuel used, the amount of nitrogen oxides generated is a function, to a certain extent, of the fuel used, but also of the conditions under which the combustion takes place. Therefore there is no univocal relationship between the emissions of nitrogen oxides and the fuel. At the very most, when one has a good knowledge of a given process (coal, heavy fuel oil, natural gas, etc. thermal power plant), it is possible to formulate an emission factor which will be used inter alia as a base reference for the advances and reductions in the emissions of nitrogen oxides which could be obtained by subsequent research and development.
The combustion of a hydrocarbon-based compound is therefore always accompanied, in addition to carbon dioxide CO2, water H2O, and nitrogen N2, by a production of nitrogen oxides. These oxides are represented by nitrogen monoxide (NO), nitrogen protoxide (N2O), and by a very small proportion of nitrogen dioxide (NO2).
From an environmental and health viewpoint, it is important to reduce their emissions therein since each of these nitrogen oxides has a significant impact:                NO participates in the phenomenon of acid rain and in the formation of tropospheric ozone;        N2O is a greenhouse gas three hundred and ten times more powerful than CO2.        
In order to reduce the emissions of NOx, processes have been developed, in particular the following two processes:                a non-catalytic process operating at high temperature, above 800° C. in the combustion chamber, this process being denoted by the acronym SNCR (selective non-catalytic reduction);        a catalytic process that operates with regard to the treatment of the flue gases, at medium temperature (300° C.-400° C.) or at low temperature (180° C.-230° C.), this process being denoted by the acronym SCR (selective catalytic reduction).        
The SCR process makes it possible to abate large amounts of NOx, but at the expense of major economic and environmental drawbacks. The more economical SNCR process does not make it possible to achieve a nitrogen oxides removal efficiency as high as the SCR process.
The objective of the invention is, above all, to provide a process for the denitrification of flue gases, of the SNCR process type, the nitrogen oxides removal efficiency of which is higher so as to tend toward the performances of an SCR process, and to have an effective, relatively economic, denitrification process with as low as possible an environmental impact.
Current Problem with the SNCR Process
The SNCR process for reducing nitrogen oxides consists in directly injecting the reducing agent into the combustion chamber in a zone where the temperature of the gaseous effluents is preferably between 850° C. and 1000° C.
By using ammonia as a reducing agent, the chemical reactions given below occur more or less simultaneously. At a temperature below 850° C., the reactions are too slow; at a temperature above 1000° C., the secondary reaction dominates with an increase in nitrogen oxides.
Main reaction:4NO+4NH3+O2→4N2+6H2O (reduction)
Undesired secondary reaction:4NH3+5O2→4NO+6H2O (oxidation)
The temperature window takes on considerable importance since, if the temperature is too high, the ammonia is oxidized which leads to a production of nitrogen oxides; if it is too low, the degree of conversion will be too low and the ammonia will be observed downstream (ammonia leak).
The reaction of the nitrogen oxides and of the ammonia, or of the urea, in water and nitrogen depends strongly on the temperature and on the residence time in the required temperature range. The temperature window for an aqueous ammoniacal solution (aqueous ammonia) lies between 800° C. and 1100° C., the optimum temperature being 930° C./960° C.
By way of comparison, the temperature window when urea is used is narrower (between 850 and 1050° C.), the optimum temperature being 960° C./980° C.
A first difficulty with the SNCR process is therefore the narrowness of the optimum temperature window.
A second difficulty consists in thoroughly mixing the gases to be treated with the reducing agent, also referred to as reactant.